Apparatus for increasing the production output of stretched flat film webs composed of thermoplastic material

ABSTRACT

This invention relates to an improvement in a method for increasing the production of stretched flat film webs of thermoplastic material in which a melt of thermoplastic material is extruded from a slot die, the melt is solidified to form a film, the film is transversely stretched, the stretched film is heat-set, divided into narrower webs, and wound up, the improvement comprising extruding the melt from a slot die wider than the die normally employed for the production of a preliminary film, which usually corresponds to the width of a transverse stretching frame, dividing the melt in the longitudinal direction into at least two films, superposing the film webs, and transversely stretching said superposed webs. The invention also relates to an apparatus for performing the method.

This is a division of application Ser. No. 590,501, filed June 26, 1975.

The present invention relates to a method for the production of at leasttransversely stretched flat film webs composed of thermoplasticmaterial, in particular of polyesters or polypropylene, and to anapparatus suitable for carrying out the method.

Methods and apparatuses for stretching flat plastic films in at leastone direction have formed part of the prior art for a long time. Thus,numerous methods for longitudinal or transverse stretching are known, aswell as methods for longitudinal-transverse stretching andtransverse-longitudinal stretching and methods for simultaneousstretching.

In the stenter methods, the melt is pressed through a slot die,solidified on a cooling drum to form a preliminary film, thenlongitudinally and/or transversely stretched, as desired, and thenheat-set, if desired, before the flat film web is wound up.

The stretching process is limited by the width of the transversestretching frame, which usually is about 5 meters, and by the maximumoperating speed of the machine, which usually is about 200 m/min, whichalso limits the production output.

The limits are generally due to constructional measurements. In order toguarantee the necessary retention time of the films in the stretchingareas, and during heat-setting, the traveling speeds would have to beincreased and the constructional measurements extended to an intolerabledegree.

From German Pat. No. 1,704,764, a process for the production ofstretched film webs of thermoplastic material is known, in which processseveral superposed film webs are stretched and, by means of pressingand/or heating before stretching, the adherence of the different filmwebs to each other is adjusted in such a manner that on the one hand thefilm webs adhere immovably to each other during the stretching process,but on the other hand they can be separated after the stretching. Theobject of this patent is a stretching process which makes it possible tostretch even those film webs the flexibility of which does not permit,or permits only under unfavorable conditions, a stretching processcarried out in known manner.

Even if multi-layer composite films are stretched, the abovementionedprocess serves in no way to increase production output of at leasttransversely stretched plastic films.

It is the object of the present invention to considerably increase theproduction output of at least transversely stretched flat film webs ofthermoplastic material.

This object is achieved by an improved method in which the melt ofthermoplastic material is extruded from a slot die, the melt issolidified to form a film, the film is transversely stretched, thestretched film web is subjected to heat-setting and is divided intonarrower webs and then wound up. The improvement is that the melt isextruded from a slot die which is wider than the die normally used forthe production of the preliminary film, usually corresponding to thewidth of the chosen transverse stretching frame and for the subsequenttransverse stretching, the melt is preferably divided in thelongitudinal direction into at least two films, the solidified film websare, before the transverse stretching takes place, superposed and thesuperposed film webs are together transversely stretched.

The considerable advantages of the method according to the invention areapparent, because the usual stretching frames may be used withoutchanging their construction, and the film output is at least twice asgreat. The die necessary for this need not be more than twice as wideand should have the corresponding extruding device and a device forsuperposing the film webs before the transverse stretching takes place.

However, the invention is not limited to the simultaneous stretching oftwo film webs, but is also useful for simultaneously stretching morethan two film webs, in which case a slot die having the correspondingwidth and several points for longitudinal division as well as acorresponding number of devices for superposing the films must beprovided. The number of webs is limited only by the width of producibleslot dies and by the thickness of the films to be transverselystretched, which should all be subjected to the same conditions in thestretching frame.

In the production of thin films in the range from about 2 to 12μ,several film webs may be stretched simultaneously whereas in the case ofthicker film webs up to a thickness of about 100μ, stretching ispreferably carried out with only two film layers.

Even if the longitudinal division basically also can be carried out whenthe preliminary film is cooled off, e.g. by means of knives, practicehas shown that it is particularly favorable and safe to divide the melt.It has proved to be particularly favorable to divide the melt in the dielip, which is preferably done by dividing wedges.

Of course, it must be taken into account in this method that each of thedivided preliminary films must corresond, as far as its volume goes, tothe quantity of material which is necessary for one finished film, afterthe separation of the two layers. This means that the extruder outputmust be increased, for example doubled.

The method according to the invention is preferably employed in theproduction of thin films of a thickness of about 2 to 12μ. However, itmay also be employed in films having a thickness of from 12 to 100μ, andmore. In the case of thin films it is particularly easy to adjust theextruder output. An operator is also able to adjust the width. It shouldbe first attempted to make use of the maximum width of the extrudedpreliminary film and to complement the rest of the width by a modifiedtransverse stretching ratio, in order to obtain the final width whichcan be produced with the machine.

However, the method according to the invention has, in addition to bedesired increase in production, two other considerable advantages. Oneis that the operational safety of the machine is improved, because it isimprobable that all the superposed film webs will break simultaneously,so that, in case one film web breaks, the others act as a support duringtransport. The other advantage is that the film webs stretchedsimultaneously and transversely also can be wound up simultaneously.

Since the thick and thin areas which are caused by the die lip arealways present, and run over a length of several 10,000 m over the sameparts of the film, they are all wound onto each other, if only one filmis wound up. This leads to distortions and stretchings which render thefilm useless for further treatment.

If the film webs treated according to the invention are wound onto eachother, the thick and thin areas are no longer wound onto each other,because each film web has its own profile.

The method according to the invention however is not limited to theproduction of transversely stretched films, but preferably also may bepreceded by longitudinal stretching, in which the flat film webs areparallel to and beside each other and are longitudinally stretched bymeans of rollers which rotate at different speeds.

It is apparent that the rollers of the device for longitudinalstretching and the cooling roller for the melt film must be adjusted tothe width of the slot die used.

The stretched film webs which are superposed according to the inventionalso may be subjected, depending on their purpose, to a knownheat-setting, which is also carried out with the film webs superposed.

After the entire treatment the film webs are usually cut to their usefulwidth, a process which is also preferably carried out while the webs arestill superposed.

After the preferred cutting process, the film webs may be separated andseparately wound, if desired, or, as described before, may be woundtogether or directly subjected to further treatment, e.g. coating orprinting.

The present invention also relates to a suitable apparatus for carryingout the method.

The simplest version thereof is composed of an extruder connected with aslot die, a cooling roller for cooling the melt and a frame fortransverse stretching, for example a frame for heat-setting, as well asa device for dividing the melt into single webs and a winding device. Adividing device, preferably for dividing the melt into at least twolongitudinal webs, as well as a device for superposing the differentwebs with edges in register, is positioned in front of the transversestretching frame.

The extruder, the slot die and the cooling roller are, as far as theirproduction output and their widths are concerned, constructed in such amanner that the amount of preliminary film produced is at least twice ashigh as the quantity usually produced for the transverse stretchingframe employed.

The device for dividing the melt into at least two longitudinal webs maybe basically arranged at any place in front of the transverse stretchingframe and in front of the device for superposing the different webs andmay be composed of corresponding knives, blades, band saws, inductivelyheated dividing devices, laser devices, or the like.

In practice, however, it has proved to be particularly suitable todivide the melt before it reaches the cooling drum, preferably in thedie. For this, dividing wedges are preferably used.

The device for superposing the different webs preferably is composed ofoblique revolving rollers over which the different webs may be guidedand thus superposed with edges in register, in order to be then guidedtogether to the transverse stretching frame.

If the film is to be stretched biaxially, it is suitable to arrange thelongitudinal stretching device, which is composed of rollers rotating atdifferent speeds, in front of the transverse stretching frame, theparallel webs which run side by side being simultaneously longitudinallystretched by rollers having the corresonding width.

If the films are to be heat-stabilized, a heat-stabilization frame islocated after the transverse stretching frame.

Before the stretched and superposed films are wound up, they may betogether divided into suitable useful widths and then wound together orseparately onto winding cores.

The present invention will be further illustrated by reference to theaccompanying drawings, in which:

FIG. 1 is a schematic plan view of an apparatus with the slot dieforming a right angle with the level of the drawing,

FIG. 2 is a view and partly a section of the front of the apparatus,approximately at the level II--II of FIG. 1,

FIG. 3 is a side view of the device for superposing from the levelIII--III of FIG. 1 and

FIG. 4 is a perspective drawing of the device for superposing togetherwith the last pair of deviating rollers of the device for longitudinalstretching.

In FIG. 1 the melt leaves the extruder 1 through the slot die 2. In theslot die there is a dividing wedge 18 (FIG. 2) which divides the melt,in this case into two melt streams 4, 4' which are solidified on thecooling roller 3 to form preliminary films. This division by means ofthe wedge 18 is only one possible embodiment. An alternative is, forexample, to effect the division after the longitudinal stretching. Thepreliminary films 4, 4' run over the roller 20 into a device forlongitudinal stretching which consists of three in-feed and dischargerollers 5a and 5b, respectively, and of the guide rollers 5c, 5d, 5e and5f. The preliminary films, which run parallel and side by side, arelongitudinally stretched by the rollers driven at different speeds.

The deviating rollers 6 and 7 are arranged behind the three dischargerollers and the longitudinally stretched webs partly wind round it (FIG.4). From there the webs are passed through a device 12 for superposing.The web 4 is then guided over the rollers 8 and 11, which can beadjusted in the bearings 13 and 15. The rollers 8 and 11, which can beadjusted to all directions, are adjusted in such a manner that the bendof the deviation is supported and that the two webs 4 and 4' aresuperposed with their edges in register, thus having a common axis(FIGS. 3 and 4). The web 4' is deviated accordingly over the rollers 9and 10 which are supported by the bearings 14 and 16. The bends of thewebs 4 and 4' are arched downwardly and upwardly, respectively. Thesuperposed webs with their edges in register are guided over acollecting roller 11' and at the same time gripped by the clips 17, 17'of the transverse stretching frame which is provided with a heatingchamber 19. This roller 11' is adjusted exactly parallel to the frameand the rest of the apparatus in order to neutralize all the angles andto make possible the exact seizure of the double web by the clips. Acomplete description of the transverse stretching frame is not included,because this is a known device.

FIG. 2 shows the slot die 2 provided with the dividing wedge 18 whichdivides the melt, before it reaches the cooling roller 3, into the twomelt webs 4 and 4'.

Apart from the deviation described, other deviations also are possible.For example, roller pairs which are arranged on top of each other orside by side may be used at the points of deviation. One web windsaround the rollers in the form of an S each time.

Whereas the first pair creates the deviation angle of the web by meansof a certain oblique position with the axes of the rollers forming theangle bisector of the deviation angle, the webs are again caused to runparallel to each other by means of the opposite adjustment of the secondpair of rollers, after the desired distance between the webs has beenreached.

As an alternative to the use of a slot die having the correspondingwidth, a method and an apparatus may be used in which at least two slotdies are arranged side by side and the corresponding number of meltstreams is extruded. The essential pOint remains the basic concept ofthe invention to produce the webs side by side and guide them in thisposition, stretch them longitudinally, if desired, superpose them beforethey are transversely stretched, and in this condition pass them on tothe clips.

It will be obvious to those skilled in the art that many modificationsmay be made within the scope of the present invention without departingfrom the spirit thereof, and the invention includes all suchmodifications.

What is claimed is:
 1. In an apparatus for increasing the production ofstretched flat film webs of thermoplastic material including anextruder, a flat slot die connected to said extruder, a cooling roller,and a transverse stretching frame,the improvement comprising a slot diewider than the die normally employed for the production of a preliminaryfilm which usually corresponds to the width of the transverse stretchingframe, an extruder having an output corresponding to the width of saidwider die, dividing means at said slot die for dividing a melt issuingtherefrom into at least two longitudinal webs, means for superposingsaid webs in register prior to entry into said transverse stretchingframe, and means for separating the superposed films in non-laminatedcondition after transverse stretching.
 2. An apparatus according toclaim 1 in which said dividing means is mounted inside said die.
 3. Anapparatus according to claim 2 in which said dividing means comprisesdividing wedge means.
 4. An apparatus according to claim 1 in which saidmeans for superposing said webs are freewheeling rollers mounted in anoblique position.
 5. An apparatus according to claim 1 including,downstream of said cooling roller, rollers for longitudinally stretchingsaid webs, said rollers being adapted to rotate at different speeds. 6.An apparatus according to claim 1 including diverging clip means forgripping said webs.